1.) Field of the Invention
The invention relates to a process for the preparation of a catalyst capable of promoting the oxidative conversion of methane into C.sub.2.sup.+ hydrocarbons with a high activity and selectivity.
2.) Background of the Related Art
There is at present no industrial-scale process which makes it possible directly to convert methane, a hydrocarbon which has a limited field of application, into higher hydrocarbons such as ethylene, which are capable of many utilizations in various fields of the chemical industry.
However, there are many catalyst systems which have been proposed within recent years, for converting methane, in the presence of oxygen, into high hydrocarbons and especially into C.sub.2.sup.+ hydrocarbons, which are mixtures of ethylene, ethane and of small quantities of hydrocarbons containing at least three carbon atoms in the molecule, this operation of conversion of methane into higher hydrocarbons being commonly called an oxidative conversion or oxidative coupling of methane.
Thus, for the selective conversion of methane into C.sub.2.sup.+ in the presence of oxygen, reference DE-A-3,237,079 proposes to employ a catalyst based on PbO and on SiO.sub.2, and reference EP-A-0,196,541 describes the use of an Li/MgO catalyst, in the formulation of which the lithium is generally introduced in the form of carbonate. Such catalysts rapidly lose their activity at the temperatures needed to activate methane, which are of the order of 750.degree. C., given that at these temperatures PbO is eliminated by sublimation and lithium carbonate is unstable and decomposes.
In reference EP-A-0,189,079, a catalyst based on rare earths in the form of oxides is proposed for performing the conversion of methane in the presence of oxygen. The results of the tests presented in this reference show that the use of a catalyst of this kind does not make it possible to reach a sufficient yield of ethylene and ethane. In fact, the selectivity for these two hydrocarbons is high at a low degree of conversion of methane, but it quickly decreases when the conversion increases. The use of the said catalyst appears therefore to be of little economic interest for an industrial application of the oxidative conversion of methane into higher hydrocarbons.
Reference WO-A-86/07,351 proposes to perform the conversion of methane in the presence of oxygen by employing a catalyst based on rare-earth oxides doped by adding oxides of metals of groups IA and IIA of the Periodic Table of the Elements, and this makes it possible to work with space velocities of the methane/oxygen mixture which are markedly higher than those envisaged in reference EP-A-0,189,079 and to attain higher degrees of conversion of methane with improved selectivities for hydrocarbons.
It has been found that a mixture of oxides of metals of groups IA and IIA and of rare-earth metals was not the most appropriate form for constituting an active and selective catalyst for oxidative conversion of methane into C.sub.2.sup.+ hydrocarbons, and that it was possible to obtain a catalyst based on the said metals exhibiting improved performance, when the catalyst preparation process passed through a stage of coprecipitation of these metals in the form of carbonates and/or hydroxycarbonates.